The technique of time-resolved waveguide modulation is applied to an optical waveguide of the conjugated polymer poly (3-hexylthiophene). This method yields values of the sign and magnitude of the complex intensity-dependent refractive index, n 2, for a guided beam at 1.06 μm (1.17 eV), upon pumping into the tail of the absorption. We find that the picosecond electronic contribution to n 2 has a negative real part, and a positive imaginary part, with ¦ n 2¦ ⋍ 1 × 10 -4 (MW/cm 2) -1, for a pump photon energy of 2.06 eV. The positive imaginary part correlates with the photoinduced absorption at 1.06 μm measured on the same sample, and consistent with earlier results on the same material, which indicate a shift of oscillator strength from the interband transition to localized absorptions in the infrared. The negative real part implies that the observed photoinduced absorption peaks at an energy below 1.17 eV. The time-resolved component of n 2 is accompanied by a slow component (> 12 ns) with positive real and imaginary parts and comparable magnitude, indicating a thermal increase in n (or equivalently the film thickness) and a long-lived photoinduced absorption.